Conventionally, in an electrophotographic image forming apparatus, a sheet on which toner is adhered is conveyed to a fixer to fix the toner on the sheet by applying heat and pressure to the sheet. The fixer comprises a heat roller and a press roller between which a nip part exists, when the front end of a sheet enters the nip part, the sheet receives an impact, which sometimes causes a fluctuation in conveyance speed of the sheet. The higher the pressure of the nip part is, the more possible the fluctuation in conveyance speed occurs.
The fluctuation in the conveyance speed is transferred to a transfer section in front of the fixer as vibration via the sheet, the toner image on the sheet is disordered due to the vibration; moreover, due to the vibration, the fluctuation in the rotation speed of a photoconductor occurs and sometimes a latent image may be disordered. Consequentially, the quality of the image on the sheet is deteriorated, for example, the image on the sheet is blurred or a white or black streak appears in the image on the sheet.
To prevent the deterioration in the quality of an image, an exemplary method is known according to which a sheet is bent when conveyed from a transfer section to the nip part of a fixer so as to prevent the transmission of the fluctuation in the conveyance speed of the sheet to the transfer section or a photoconductor.
However, as the conveyance speed of the sheet passing through the fixer is decreased or increased to a fixed value, if the hardness of a sheet is changed according to the basis weight of the sheet, a different environment and the like, the bending amount of the sheet will not be a fixed value. For example, there is a problem that an image blur and the like is generated, or the sheet is bent than expected such that the image on the sheet is rubbed by the conveyance section and therefore the deterioration in the quality of the image is generated when a bending amount cannot be obtained at which the vibration won't be transferred.